Sources find it incomprehensible that exposing spy agency
techniques for tracking terrorists has been argued to be in the
public interest.

Leaks from Snowden are known to contain at least 58,000
Government Communications Headquarters
(GCHQ) files and it is feared there could be many
more. It also unclear whether foreign states have had
access to the documents and it is understood the
Guardian continued to expose the information despite
pleas from the Government not to reveal intelligence
techniques.

It is believed to be the worst leak of British intelligence
files and to have caused the greatest damage.

In his first speech since becoming head of MI5 in April, Mr
Parker did not specifically name Snowden or the
Guardian.

But he said: “It causes enormous damage to make public the
reach and limits of GCHQ techniques.

Such information hands the advantage to the terrorists. It is
the gift they need to evade us and strike at will.
Unfashionable as it might seem, that is why we must keep
secrets secret, and why not doing so causes such harm.

“That margin gives us the prospect of being able to detect
their plots and stop them. But that margin is under attack,” he
said.

He said reports from GCHQ were “vital to the safety of this
country and its citizens,” adding: “We are facing an
international threat and GCHQ provides many of the intelligence
leads upon which we rely.”

Mr Parker said the U.K. is already facing its most complicated
and unpredictable terror threat and that it was “getting
harder” for his agents to protect against the diverse dangers.

With the spread of an al-Qaeda threat to more and more
countries, the continued danger of Irish terrorism, the
emergence of the lone wolf fanatic and advances in technology
and cyber warfare, MI5 is now “tackling threats on more fronts
than ever before,” he said.

In the speech to the Royal United Services Institute in London,
Mr Parker said: “Our task is getting harder. The threats are
more diverse and diffuse. And we face increasing
challenges caused by the speed of technological change.”

And he warned: “It remains the case that there are several
thousand Islamist extremists here who see the British people as
a legitimate target.”

Among those are Britons, numbering in the low hundreds sources
say, who have traveled to Syria, which is now a hotbed of
extremism and terror groups, and since returned home.

The spy chief said: “For the future, there is good reason to be
concerned about Syria. A growing proportion of our
casework now has some link to Syria, mostly concerning
individuals from the U.K. who have traveled to fight there or
who aspire to do so.”

While the threat of a large scale terror outrage may have
diminished it has not been removed, he said, while there is a
growing risk of smaller attacks or individuals acting on their
own. Since 2011, a total of 330 people have been convicted
of terrorism-related offenses in Britain. There is also
the threat to Britons around the world, such as the attack on
the In Amenas gas facility in Algeria and the recent Westgate
shopping center outrage in Nairobi, Kenya.

“Overall, I do not believe the terrorist threat is worse now
than before. But it is more diffuse. More complicated.
More unpredictable,” he said. There have been one or two
major terror plots in the U.K. every year since 2000 and that
pattern is “unlikely to change.”

And it was impossible to protect the public 100 percent, he
said, adding “life is not the movies.” Because of its nature
and terrible consequences, there was an expectation that there
should be “zero” attacks but no crime can have such a target.

In a clear defense of any potential intelligence failings by
MI5, Mr Parker also stressed there was a difference between
“knowing of someone and knowing everything about them.”

“The idea that we either can or would want to operate intensive
scrutiny of thousands is fanciful,” he said. “This is
not East Germany, or North Korea. And thank goodness it's not.”

He also made a defense for extended powers to monitor modern
communications, the subject of recent controversy, saying “we
cannot work without tools.”

He said the idea that the agencies would use such powers to
monitor everyone’s private lives was “utter nonsense.”

Explaining why he made a public speech, he said it was
important for spies to occasionally step out of the shadows to
explain to the public the threats they face.

A Guardian News and Media spokeswoman said: "A huge
number of people — from President Obama to the U.S. director of
national intelligence, James Clapper — have now conceded that
the Snowden revelations have prompted a debate which was both
necessary and overdue.

The President has even set up a review panel and there have
been vigorous discussions in the U.S. Congress and throughout
Europe. Such a debate is only worthwhile if it is informed.
That is what journalism should do.

Henry Porter, a columnist at the Observer, the
Guardian's sister newspaper, said Sir Andrew was
"wrong" to suggest leaks have put lives at risk. He said
that he has lost confidence in the Intelligence and Security
Committee, the body of MPs and peers which oversees the
security services.

Mr Porter said: "He's wrong [to say the Guardian put
security at risk]. The people who released and let go of these
documents were the NSA in America. That's where these leaks
took place.

What we have done is shown how much surveillance we are under.
We don't have sufficient oversight. I don't have that
confidence because of the behavior of the intelligence and
security committee over the last few months, which has steadily
come out in favor of the intelligence services.

Snowden, 30, was a CIA analyst based in the U.S. National
Security Agency, who provoked one of the biggest intelligence
leaks in American history. He used his position to access
and steal thousands of classified documents on U.S. and related
British spy programs.

The leaks were revealed in a series of articles in the
Guardian newspaper in June. He fled the U.S. and
is currently being sheltered in Russia.